Where to Find the World's Clearest Lake

In this week's Maphead, Ken Jennings unearths the location of the world's clearest lake. It's located in the Alps…but not the ones you think.

Travelers love crystal-clear water, but where would you head to find the lake that's the world's very clearest? A 2011 study found that the world's purest, most pristine freshwater is found in the Alps—but hold the Ricola and the lederhosen for a minute. The aptly named Blue Lake is actually found on the exact opposite side of the world from Switzerland. To see the world's clearest water, you'd have to visit the Southern Alps, the range that runs the length of the South Island of New Zealand.

Blue Lake is as clear as distilled water.Blue Lake (or Rotomairewhenau, if you prefer a less boring name) is found in Nelson Lakes National Parks, high in the New Zealand backcountry. A 2011 optical study by New Zealand's National Institute of Water and Atmospheric Research found that underwater objects are still visible through 80 meters of Blue Lake water. Only certain pristine Pacific waters near Easter Island have received optical scores as high as Blue Lake.

What's the secret of Blue Lake's clarity?It's fed entirely by glacial water from a much larger lake, Lake Constance, less than a mile to the south. Lake Constance sits just above the mountain beech treeline, which keeps its waters free from most plant and animal material. And thanks to a long-ago landslide, Lake Constance's water filters through glacial moraine before flowing into Blue Lake almost entirely impurity-free. The lake drains completely every 24 hours, so water doesn't stick around long enough to stagnate. As a result, the water is a brilliant blue-violet color—and bitterly cold year-round.

But you can't take a dip in the lake.Before you pack your bags for New Zealand, be warned: you can get to Blue Lake in a two-day hike from the park entrance, but you won't be able to put a toe in the lake, much less swim or dive. The local Maori tribe considers the lake to be tapu, or sacred. These underwater photos of Blue Lake, taken in 2013 by a Danish photographer with special permission, are as close as you'll get.

If you want to swim in unearthly clear waters, there are other possibilities.Russia's Lake Baikal, the world's deepest lake, is certainly a contender. For Americans, Crater Lake may be a more conveniently located candidate. Crater Lake, in an extinct volcanic caldera in the mountains of southern Oregon, is so clear that objects are visible 139 feet below the surface. Water that clear doesn't filter out the ultraviolet in sunlight, and scientists think that's why there's almost no ecosystem in Crater Lake: the UV kills off the plankton. It's water that's actually too clean for nature to handle.

Explore the world's oddities every week with Ken Jennings, and check out his book Maphead for more geography trivia.